This invention relates to power sensors, and is specifically concerned with sensors which measure the power of an applied microwave signal. Power at microwave frequencies can be determined as a measure of the rise in temperature of a load which absorbs the power and it has been proposed to utilize a power absorption element incorporating a thermocouple as the load in which the magnitude of an output signal is related to the incident microwave power. When implemented as a semiconductor arrangement, the power absorption element is termed a semiconductor thermoelectric element.
The relationship between output signal and applied power is a non-linear one, and it is usually necessary to provide some form of calibration for the power sensor by applying known reference power levels, and noting the resulting output signals, the values of which preferably cover the range of power and frequency values to be encountered. Once the calibration has been completed, it is necessary to subsequently apply a reference power level to the sensor so that either its output signal can be brought to a predetermined value or so that a correction factor can be applied. This procedure is usually necessary before a series of measurements are to be performed to allow for drift in the operating characteristics of the power sensor. As it becomes desirable to extend the operation of power sensors to higher frequencies at which waveguide transmission techniques are required, a limiting factor is the difficulty encountered in providing a suitable reference power level having a sufficiently accurate and stable value. The present invention seeks to provide an improved power sensor.